Free soundfonts (and a whole bunch more stuff) at sf2midi.com (Registration required).
I can tell you that wading through free soundfont libraries online is a very, very time consuming endeavor but can turn out to be very, very rewarding when you find a good one. The selection doesn’t seem that deep at sf2midi but we keep wading.
Hammersound is the classic free site. Their servers seem to fade out sometimes so keep trying.
[UPDATE]: Thanks Gagarin for pointing us back to sf2midi who has done a nifty redesign of the site and quite an uptake in SF2 files! As of this writing there over 2,200 sound fonts and it is now easier than ever to find what you may be looking for. Even though they are still missing some ‘classics’ (like the Campbell strings sections) they are an essential stop.